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28. Find the sum of first 51 terms of an AP whose second and third terms are 14 and 18respectively.Sobot |
Answer» Answer:5610 Step-by-step explanation:Let first term be a and common DIFFERENCE be d We have : a + ( 2 - 1 ) d = 14 a + d = 14 a = 14 - d .... ( i ) a + ( 3 - 1 ) d = 18 a + 2 d = 18 a = 18 - 2 d ... ( ii ) From ( i ) and ( ii ) 18 - 2 d = 14 - d d = 4 a = 14 - d a = 10 Now : S₅₁ = 51 / 2 ( 20 + 50 × 4 ) S₅₁ = 51 ( 10 + 100 ) S₅₁ = 5610 |
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